A downhole well logging tool typically has one or more sensor sub-units. Each sensor sub-unit has a cylindrical section of pipe enclosing one or more sensors and a micro-controller having non-volatile memory for storing a log of sensor data. For example, the sensor data indicates inclination, temperature, pressure, vibration, magnetic field, and gamma rays.
Although a sensor sub-unit may contain a battery for powering the sensor sub-unit, it is not uncommon for the sensor sub-unit to be powered by a battery or mud turbine in a separate sub-unit that is shared among the sensor sub-units. For example, a typical downhole sensor sub-unit without a battery may operate at temperatures up to 230 degrees Centigrade, which is higher than the permissible operating temperatures of common batteries. For reliable operation at such high temperatures, a sensor sub-unit may be powered by a mud turbine instead of a conventional battery. Consequently, it is convenient to build sensor sub-units without batteries. This permits downhole well logging tools to be configured from selected sub-units to suit a wide range of downhole environments.
A conventional log of data from a sensor is a time series of periodic samples of data from the sensor. In a sensor sub-unit, each sample of the data is taken at a particular time in response to a clock signal in the sensor sub-unit. Each sample of sensor data is recorded in a sensor log. For each sample of sensor data, an associated time value is also recorded in the sensor log. The association of the time value with each sample is used to determine the depth of the sensor in the wellbore at the time that the sample was taken. Typically this is done by correlating the real time of the sample with the depth as a function of time as determined by tracking of the movement of the logging tool in the wellbore. For logging while drilling, the movement of the logging tool in the wellbore can be tracked by observation of the drill string sections as the drill string sections are lowered into the well head or a drilling riser. For wireline logging, the movement of the logging tool in the wellbore can be tracked by measuring the play-out of the wireline from a spool as the logging tool is lowered into or raised from the wellbore.
By correlating the sensor samples with depth, a view of the formation surrounding the wellbore can be constructed from the sensor samples. By precisely correlating sensor samples with depth, it becomes possible to combine sensor data from different logging tools used at different times in order to provide a more detailed view of the formation surrounding the wellbore. Without precise or accurate real-time values for the sensor samples, there can be a loss of precision or accuracy in the constructed view of the formation surrounding the wellbore.